The present invention relates to an apparatus capable of high-speed detection of the position of incidence of particle beams such as those of electrons and photons.
Microchannel plates are extensively used to detect and multiply charged particles such as single electrons or photons such as those of UV light, X-rays and gamma-rays. The position at which these particles are incident upon the microchannel plate must be in exact alignment with the position where they emerge from the plate and this alignment is established for each of the channels. To this end, attempts are being made to detect the position of incident particles encountering the microchannel plate by combining it with an appropriate device such as a semiconductor position sensitive device (PSD) using the PN junction of a silicon semiconductor, a "resistive anode" device using a resistor, or a wedge-and-stripe anode using a combination of a stripe electrode and a wedge electrode. In order to facilitate the processing of the output from the microchannel plate in subsequent electronic circuitry, single charged particles, say, electrons, are preferably multiplied by factors of 10.sup.6 -10.sup.7 or even more. A method currently employed to meet this need is to perform multiplication at more than one stage through two or more microchannel plates placed in tandem.
FIG. 10 is a diagrammatic cross section of a conventional apparatus for detecting the position of incidence of photons by converting them to photoelectrons. A photocathode 1 is formed on the inner side of the surface of a vacuum chamber 5 where photons are to be encountered. The individual photoelectrons emitted from the photocathode 1 are multiplied as they pass through two microchannel plates 101 and 102. As already mentioned, the factor by which these electrons are multiplied is in the range of 10.sup.6 -10.sup.7. The multiplied electrons then encounter a two-dimensional incident position detector 103 that is formed of a PSD located in a face-to-face relationship with the output surface of the microchannel plate 102.
FIG. 11 is a sketch of the two-dimensional incident position detector PSD as seen from the surface of the chamber 5 where photons are to be encountered. The two-dimensional incident position detector 103 has a uniform resistive surface and two electrode pairs X-hd 1-X.sub.2 and Y.sub.1 -Y.sub.2 that surround this resistive surface. Each of the electrodes produces an electric current that contains information on the position of incidence of electrons. If the currents flowing out of the electrodes X.sub.1, X.sub.2, Y.sub.1 and Y.sub.2 are written as ix.sub.1, ix.sub.2, iy.sub.1 and iy.sub.2, respectively, the coordinates, x and y, of the point of incidence are given by the following equations, assuming that the coordinates of the center of the area surrounded by the four electrodes are (0, 0): EQU x=k1(ix.sub.1 -ix.sub.2)/(ix.sub.1 +ix.sub.2) EQU y=k2(iy.sub.1 -iy.sub.2)/(iy.sub.1 +iy.sub.2)
where k1 and k2 are constants. These equations are derived on the basis of the assumption that, as shown in FIG. 12, the amount of the current appearing at each terminal is in inverse proportion to the value of resistance offered by the distance from the point of incidence of a particle to that terminal.
The time response characteristics of the abovedescribed apparatus for detecting the position of incidence of particle beams provide an important factor in the determination of the counting rate of single incident signals. The response of the detector is preferably as quick as possible and this is also true for the case where the position signal is to be fed back and used as a control signal. Consider, for example, the case of controlling muon beams. Muons, as they pass through a thin carbon film, will emit secondary electrons. The secondary electrons, which are much smaller in mass than muons, will travel much faster when accelerated. By detecting the position of these electrons at which they encounter a position detector that employs microchannel plates, one is able to identify the position where the muons passed through the carbon film. If the located position is different from the desired position, a signal is fed to deflecting electrodes behind the carbon film (this is a feedback operation) so that the muons will be deflected to the desired position. The operating principle of this control is that secondary electrons which are much lighter than muons are accelerated to travel at such high speeds that the position of their incidence can be rapidly detected by the microchannel plates. In order for the intended control to be performed successfully, the time required for the control including the detection time must be shorter than the reciprocal of the travelling speed of muons.
The PSD type incident position detector which employs the PN junction of a semiconductor has a large capacity and its time constant is as large as several hundred nanoseconds. The resistive anode type detector can be designed to have a capacity that is about one order of magnitude smaller than that of the PSD type but its time constant is only a little smaller than 100 nanoseconds. The wedge-and-stripe type detector has the potential to be operated with a shorter response time than the PSD and resistive anode types, but has the disadvantage of having complex structure. If the structure of a position detector is complex, the calculation of current distribution also becomes complex and hence time-consuming, and this eventually leads to an increased overall response time.
Under the circumstances described above, none of the prior art position reading apparatus that combine conventional types of microchannel plate with other incident position detectors have succeeded in attaining a quick response faster than 10 nanoseconds.